The present invention generally relates to the television display art. More particularly, the present invention relates to display quality and control of associated video output stages for television displays.
Video output amplifiers typically utilize one or more drive transistors, which are responsive to video signals from a video source and provide high voltage output signals, to drive associated cathodes of an appropriate CRT display. Typical color televisions contain three channels--R, G and B--for providing color video signals. The three color channels form parallel processing networks, and each is identical to the others.
In some instances, the input video signal will be of a sufficient magnitude that it will drive a drive transistor within the video output amplifier for one or more of the three color channels into saturation. The resulting video output signal may be viewed on an oscilloscope which will show a clipping at the video signal peaks. Once a drive transistor enters saturation, a delay period will be introduced at the time when the video signal drops in magnitude below that required to drive the transistor into saturation. The storage charge within the transistor, which is built up when it is in saturation, creates this finite interval, which is required to dissipate the storage charge after the saturating, biasing signal is removed. As a result, the displayed video picture will exhibit flaring associated with the particular peak video signal. That is, bright objects, e.g., a human nose or ear in sunlight, a gold bracelet, waves in pool water, will become blurred in the direction of the raster scan as the video signal pulse representing that image will be spread (its termination delayed) by driving the driver transistor into and out of saturation.
A previous method for eliminating this visual distortion hard clamps the output voltage at the collector of the drive transistor, usually with a diode forward biased between a selected clamping voltage source and the drive transistor collector. This limits the output voltage to the level of the clamping voltage source (plus the negligible diode junction voltage drop) and prevents the drive transistor from going into saturation. However, it also limits detail in the resulting picture by limiting the video output signal level without regard to any characteristics of the input signal, except its magnitude.
Accordingly, a principal object of the present invention is to provide a circuit and a technique which generally overcomes the deficiencies of the prior art.
A more particular object of the present invention lies in providing circuitry and a technique for preventing flaring in displayed television pictures.
A further particular object lies in providing a circuit and technique for preventing flaring in displayed television pictures while maintaining picture detail and optimizing costs.
In accordance with the stated objects and other objects apparent from the following description, the present invention provides a current limiting circuit at the input to a video output driver circuit, such as a typical drive transistor. Feedback from the drive output controls the current into the drive circuit, and the current limiting circuit maintains the input level below that which would cause the drive transistor or other video output drive circuitry to go into saturation for undesirable time periods. Transient response circuitry in parallel with the current limiting circuit allows the output drive circuitry to respond to rapid swings in video input signals, without canceling the effects of the current limiting circuit. In this fashion, the displayed video picture does not exhibit flaring while maintaining a high level of detail.